This invention relates to weld metals with superior low temperature toughness for joining high strength, low alloy steels. This invention also relates to welding consumable wires and welding methods for producing such weld metals. Weld metals produced by the welding consumable wires and welding methods of this invention have microstructures that provide superior strength, toughness, and hydrogen cracking resistance. The welding consumable wires and welding methods of this invention are particularly well suited for mechanized field girth welding of high strength steel linepipe using the gas metal arc welding process to construct pipelines.
Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
For industries that utilize steel structures, e.g., oil and gas, chemical, ship building, power generation, etc., it has become apparent that the selected use of high strength, low alloy (HSLA) steels is desirable. As used herein, xe2x80x9chigh strength, low alloy (HSLA) steelxe2x80x9d includes any steel containing iron and less than about 10 wt % total alloy additives and having a yield strength of at least about 550 MPa (80 ksi). Utilizing a HSLA steel can result in lower costs for a structure due to the lower weight of the structure compared to that of the same structure built of a lower strength steel. Also, the use of HSLA steels can enable a structure to be built that could not practicably be built using a lower strength steel because very thick material would be necessary to provide structural strength, resulting in unacceptably high weight.
However, utilizing HSLA steels in some of the aforementioned structures may have certain drawbacks. Many commercially available HSLA steels are limited in their use, as compared to lower strength steels, particularly in fracture critical applications, because of the limited toughness (thus, limited defect tolerance) of their weldments. (See Glossary for definition of fracture critical.) Toughness in steel weldments may be considered from the standpoint of the ductile-to-brittle transition temperature (DBTT) as measured by the Charpy V-notch test, from the magnitude of the absorbed Charpy V-notch energy at specific temperatures, or from the magnitude of the fracture toughness at specific temperatures as measured by a test like the crack tip opening displacement (CTOD) test or the J-integral test; all of these toughness testing techniques being familiar to those skilled in the art. (See Glossary for definition of DBTT.)
Another potential drawback associated with the use of HSLA steels is the susceptibility for hydrogen cracking in their weldments. As the strengths of weld metals increase, their alloy contents typically increase, which creates higher hardenability and a tendency for transformation to martensite. The increased presence of martensite in higher strength weld metals combined with the higher residual stresses in higher strength weldments, generally leads to greater sensitivity for hydrogen cracking, as compared to lower strength weldments. In order to decrease the likelihood of hydrogen cracking in the weldments of HSLA steels, the steels are often preheated prior to welding, which can increase fabrication costs.
In addition to commercially available HSLA steels, new HSLA steels with superior strengths, e.g., yield strength of at least about 690 MPa (100 ksi), preferably at least about 760 MPa (110 ksi), more preferably at least about 828 MPa (120 ksi), and even more preferably at least about 896 MPa (130 ksi), and most preferably at least about 931 MPa to 966 MPa (135 to 140 ksi) are currently under development. For example, see International Application nos. WO 99/05336, WO 99/05334, WO 99/05328, WO 99/05335, and WO 98/38345. These new HSLA steels are particularly well suited for manufacturing high strength linepipe for constructing pipelines. For pipeline applications, the girth welds used to join individual linepipe segments preferably provide a high level of toughness due to the fracture critical nature of their service. Additionally, in certain environments, e.g., in arctic applications, the required girth weld toughness may need to be achieved at ambient temperatures as low as about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.), or even as low as xe2x88x9260xc2x0 C. (xe2x88x9276xc2x0 F.). Therefore, in order to utilize commercially available HSLA steels, or the new HSLA steels currently under development, at low ambient temperatures, welding consumable wires and welding methods that provide weld metals and weldments with adequate strength, hydrogen cracking resistance, and, most importantly, toughness, at such temperatures are required.
Broadly speaking, there are two classes of welding wires and, therefore, weld metals, that are currently available for the purpose of joining HSLA steels with yield strengths in the range of about 690 MPa (100 ksi) to about 931 MPa (135 ksi). The first class of weld metal is commonly referred to by an acronym related to its microstructure, xe2x80x9cLCBFxe2x80x9d, which stands for low carbon bainitic ferrite. This type of weld metal is described in U.S. Pat. No. 5,523,540. The second class of weld metal is the martensitic type, which is also described in U.S. Pat. No. 5,523,540.
The LCBF weld metals were invented as an improvement over the martensitic type for welding of naval hull materials. One goal was that the LCBF microstructure be producible over a wide range of welding heat inputs. Examples of welds made at heat inputs from about 1.2 kJ/mm (30 kJ/in.) to 5 kJ/mm (127 kJ/in.) are given in U.S. Pat. No. 5,523,540. It is noted in U.S. Pat. No. 5,523,540 that it is necessary to provide relatively fast cooling rates to ensure that the martensitic-type weld metals transform completely to martensite. However, when applying a wide range of cooling rates to the LCBF weld metals, the microstructure transforms entirely to bainite, and martensite is avoided. Another goal of the LCBF weld metals is avoidance of hydrogen cracking without the requirement for preheating to drive off hydrogen. This permits a cost savings during fabrication. Meeting these goals for welding naval hull materials places certain demands on the chemistry for the LCBF weld metals, particularly on the carbon content. The LCBF weld metals described in U.S. Pat. No. 5,523,540 are limited to a maximum of 0.05 wt percent carbon, primarily to avoid martensite formation. It is generally believed that the LCBF microstructure is more stable over broad heat input ranges and that it is more resistant to hydrogen cracking than martensite.
In contrast to naval hull welding where avoidance of preheating is desired, preheating is routinely used in pipeline girth welding even for nominally low alloy grades of steel like API 5L X-65. Because of the fracture critical nature of each girth weldment and the expense associated with pipeline repairs, avoidance of hydrogen cracking in pipeline girth weldments is desirable. The use of preheating in pipeline girth welding is often seen as necessary to avoid or minimize hydrogen cracking that may otherwise occur under rugged field conditions that can result in less than optimal cleanliness. Pipeline construction using mechanized equipment can proceed at a rate of 100 to 400 welds per day (depending on the equipment employed and whether construction is onshore or offshore). Because hydrogen cracking can occur more than one or two days after welding, costly remedial action can result from the occurrence of this type of cracking during pipelaying. Thus, in the pipeline industry, welding preheat is seen as relatively cheap insurance to avoid hydrogen cracking and associated field repairs. This is particularly the case for offshore pipelines where the welds become essentially inaccessible soon after welding, and it is more cost effective to apply moderate preheat than it is to xe2x80x9cpick-upxe2x80x9d the constructed offshore line and conduct a repair.
With respect to structural integrity, each girth weld in a gas pipeline is fracture critical. Leaks result when any weldment defect penetrates or propagates through the entire pipeline wall. In this case, the pipeline fails to perform its intended function. For naval hulls, however, there is a greater degree of structural redundancy. Very few welds are fracture critical to the extent that if one fractures, the ship will fail to perform its intended function.
Concerning heat input demands, naval hull welding is conducted with a wide range of heat inputs, whereas field pipeline girth welding places natural limitations on the welding procedure. Field pipeline girth welds require all-position welding on relatively thin material (typically, from about 8 mm (0.3 in.) to about 25 mm (1 in.) wall thicknesses). Many naval hull welds are made on thick sections (up to about 50 mm (2 in.)) in the flat position where relatively high heat inputs can be used. The all-position demands of field pipeline welding restricts the heat input to relatively low levels.
While the LCBF weld metal described in U.S. Pat. No. 5,523,540 may be suitable for joining HSLA steels for use in naval hulls, the LCBF weld metal is not optimal for the girth welding of pipelines whereby the desired yield strength is at least about 690 MPa (100 ksi). The heat input and preheating requirements for pipeline girth welding are distinctly different from the requirements for welding of naval hull steels. A need exists for welding techniques that generate weld metals having yield strengths in excess of at least about 690 MPa (100 ksi) and superior low temperature toughness, particularly when the welding heat input is relatively low and moderate preheat is applied. Such welding techniques would be particularly well suited for mechanized pipeline girth welding.
Therefore, an object of this invention is to provide weld metals for joining HSLA steels, which weld metals possess distinct microstructural features that are different from the LCBF and martensitic types of weld metals and provide superior combinations of toughness at low temperatures, high strength, and hydrogen cracking resistance, particularly when used for mechanized pipeline girth welding. A further object of this invention is to provide welding consumable wires and specific welding methods for producing such weld metals. Further objects are made apparent by the following description of the invention.
Consistent with the above-stated objects of the present invention, weld metals suitable for joining HSLA steels are provided. Weld metals according to this invention comprise iron, about 0.04 wt % to about 0.08 wt % carbon; specified amounts of manganese, silicon, molybdenum, nickel, and oxygen; and at least one additive selected from the group consisting of zirconium and titanium. The microstructure of weld metals according to this invention comprises from about 5 vol % to about 45 vol % acicular ferrite and at least about 50 vol % lath martensite (including auto-tempered and tempered lath martensite), degenerate upper bainite, lower bainite, granular bainite, or mixtures thereof. The balance of the microstructure may include ferrite, upper bainite, pearlite, or mixtures thereof. Also, welding consumable wires and welding methods are provided for producing the weld metals of this invention.
Weld metals produced according to this invention are particularly well suited for field girth welding of HSLA steel linepipe, particularly when the mechanized gas metal arc welding (GMAW) process is used. The microstructure of these weld metals provides high strength, good hydrogen cracking resistance, and superior low temperature toughness suitable for many cold weather applications down to about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.), or even as low as xe2x88x9260xc2x0 C. (xe2x88x9276xc2x0 F.). Another advantage of this invention is that hydrogen cracking can be avoided when using these weld metals with preheats of less than about 150xc2x0 C. (302xc2x0 F.), and more preferably less than about 100xc2x0 C. (212xc2x0 F.). These weld metals are particularly well suited for relatively low heat input welding where moderate preheat is applied, for example, the mechanized field girth welding of HSLA steel linepipe using the gas metal arc welding (GMAW) process.
Weld metals according to this invention have DBTTs, as measured from a Charpy energy versus temperature curve, of lower than about xe2x88x9250xc2x0 C. (xe2x88x9258xc2x0 F.), preferably lower than about xe2x88x9260xc2x0 C. (xe2x88x9276xc2x0 F.), and more preferably lower than about xe2x88x9270xc2x0 C. (xe2x88x9294xc2x0 F.). With respect to the upper shelf energy of the Charpy transition curve, these weld metals have at least about 100 joules (J) (75 ft-lbs), preferably greater than about 135 J (100 ft-lbs), and more preferably greater than about 170 J (125 ft-lbs). With respect to fracture toughness, CTOD testing of these weld metals produces values of at least 0.10 mm, preferably at least 0.15 mm, more preferably at least 0.20 mm, and even more preferably at least 0.25 mm, and most preferably at least 0.30 mm at a test temperature of 0xc2x0 C. (32xc2x0 F.), preferably at about xe2x88x9210xc2x0 C. (14xc2x0 F.), more preferably at about xe2x88x9220xc2x0 C. (xe2x88x924 xc2x0 F.), even more preferably at about xe2x88x9230xc2x0 C. (xe2x88x9222xc2x0 F.), and most preferably at about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.). Also with respect to fracture toughness, these weld metals produce JIC values of at least about 125 N/mm (0.7 ksi-in), preferably at least about 175 N/mm (1.0 ksi-in), more preferably at least about 225 N/mm (1.3 ksi-in) at a test temperature of about xe2x88x9210xc2x0 C. (14xc2x0 F.), preferably at about xe2x88x9220xc2x0 C. (xe2x88x924xc2x0 F.), more preferably at about xe2x88x9230xc2x0 C. (xe2x88x9222xc2x0 F.), and most preferably at about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.). (See Glossary for definition of JIC values.)